Start seeds indoors to give wildlife-friendly plants with long growing seasons a head start

02-01-1999
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Kelly Hartley

THE WEATHER OUTSIDE may be frightful, but you can still exercise your green thumb during winter. "This is the best time to plan your garden and decide what you want to grow," says Maureen Heffernan, director of public programs at the Cleveland Botanical Garden. It's also a good time to start planting seeds—indoors, that is.

Many native plants have long growing seasons; they generally will not flower from seed during the first year unless they get a head start. Wildlife-attracting perennials such as blazing star, purple coneflower and bee balm are just three examples; their seeds need to be sown between 8 and 10 weeks prior to outdoor planting.

Getting Started

Although seeds are traditionally started in flats, they will thrive in any container with drainage holes and a depth of at least 2 inches (allowing for root growth and vital air circulation). "If you are going to reuse containers, be sure that you clean them first with soapy water so diseases won't affect the seedlings," says Heffernan, who has done extensive research on various seed-propagation methods.

"Where most folks go wrong," adds the horticulturist, "is in not using the right soil mix." Gardeners should purchase a sterilized seed-starting mix at a local store or make their own. To do the latter, Heffernan recommends combining two equal parts of vermiculite and perlite or three equal parts of vermiculite, perlite and potting soil. Standard potting soil and soil from a garden are generally too compact to allow for proper seedling growth. They also may harbor diseases.

To sow your seeds, moisten the soil mix and spoon it into your seed-starting containers. Then read the instructions on the seed packet to determine the appropriate planting depth. Plant more seeds than you think you will need; some seeds will not germinate and some seedlings invariably will be lost during transplanting.

After your seeds are positioned, place clear plastic covers over the containers to prevent moisture loss. Water and moderate temperatures are essential for seed germination. Soil should be kept warm (70 to 75 degrees Fahrenheit) and moist, but not soggy. One way to maintain consistent warmth is to place the containers on top of a refrigerator. Heating mats can also be used.

"Read seed packets carefully," Heffernan says. "Pay attention to any special growing tips." Some seeds, such as columbine, require light to germinate. Others, such as great blue lobelia, require a multi-week chill period before they are sown. Seeds with hard outer coats, such as wild senna, benefit from scarification prior to planting: You may have to nick them with a file, rub them across sandpaper or drop them into hot water to soak overnight.

Proper Nourishment

As soon as the seedlings emerge, remove the plastic covers from the containers to prevent heat buildup from harming the young plants. But continue to provide the seedlings with proper nourishment. The basic requirements for growing healthy plants indoors include:

Light: Seedlings generally need 14 to 18 hours of light a day—without it they tend to grow leggy—which may require supplementing natural light (ideally a sunny, south-facing window) with artificial lighting. A fluorescent bulb is a good, inexpensive choice. Though gardeners may be tempted to expose small seedlings to light around the clock, Heffernan stresses the importance of giving plants rest (i.e., darkness)—something they will enjoy daily outdoors.

Heat: Seedlings require less heat than germinating seeds, growing best at temperatures of about 65 degrees Fahrenheit during the day and about 60 degrees at night. "When seedlings grow accustomed to too much heat, they become 'babyfied'—they get spoiled," Heffernan says. "There is succulent growth, but plants become weak."

Water: Keep seedlings moist, but be careful not to drown or disrupt them. Watering from the bottom (filling trays under plant containers with water) works best; it directs moisture to the root zone and prevents puddling on the surface—a condition that could lead to damping off, a disease that causes seedlings to rot. Plants should be allowed to sit in water for no more than five minutes. Excess water that remains in trays after the soil has become saturated should promptly be removed.

Fertilizer: Young plants like food, but too much can harm them. Heffernan recommends applying a liquid fertilizer at a strength of one-quarter the prescribed amount. Use fish emulsion or other organic products.

If your containers become overcrowded, thin out the seedlings. This allows the more vigorous plants to grow without competition. Rather than pulling out unwanted seedlings and risking damage to the roots of the plants that will remain growing, Heffernan suggests that gardeners use small scissors to cut off the extras at soil level.

Transplanting Tips

Plants not already in individual containers should be transplanted once two full sets of leaves have formed. Like the thinning process, this will stimulate the growth of more roots and improve needed air circulation. "Very gently lift the plant, being careful not to crush the stem cells," Heffernan says. "Transfer it to the new container where a transplant hole is already in place." Handle the leaves rather than the more fragile stem.

Before moving your seedlings to their permanent garden spot, let them become acclimated to the outdoors. This process is known as hardening off. Set the plants outside for a few hours each day for a period of five to seven days, gradually increasing the amount of time you expose them to the elements—sun, wind and cooler temperatures.

When you're ready to plant outdoors, water the seedlings (this helps keep root balls intact) and the area where they will be planted. Choose a cloudy day for this transition to prevent the sun's heat from causing plant wilting.

"Plants do better when they don't have to stay on hold—when there is one continuous growth cycle," says Bruce Butterfield, director of research at the National Gardening Association. "It's easy to get excited about growing seeds indoors, but it's important not to start too early," he adds. "Make certain your plants have a place to go." Gardeners should know when they can plant outdoors (the frost-free date for their area) and plan accordingly.